Sulfur dioxide (SO.sub.2) stack emissions have been identified as a contributor to the formation of acid rain and, accordingly, the detection, measurement and control of such emissions are increasingly coming under scrutiny and regulation by various governments. Certain industries particularly are being more closely scrutinized since they are more apt to produce SO.sub.2 emissions as a byproduct of the processes they engage in to produce their products. For example electric utilities and industrial power plants are a source of such SO.sub.2 emissions due to the fact that they burn fossil fuels. Such carbon based fuels are especially in need of emissions monitoring because of the fact that in addition to SO.sub.2, other pollutants are also emitted such as the carbon based pollutants CO and CO.sub.2. Certain electric utility companies which utilize high sulfur coal in their coal fired power plants have been forced to install expensive scrubbing equipment and monitoring analyzers to insure compliance with government standards. Additionally, it is essential that such equipment operate continuously and reliably with a minimum of maintenance.
For such monitoring or analysis of multiple pollutant gas stack emissions, it has become necessary to employ costly and complicated equipment to measure for such a variety of constituents and to the precise concentrations required by the various government regulatory agencies. One example of such a measurement or analyzing device used for the measurement of gas constituents in combustible environment is a spectrophotometric device which would utilize either ultraviolet or infrared light to determine the amount of SO.sub.2 present by the amount of radiation absorbed at particular wavelengths specifically associated with SO.sub.2. Additionally, the use of such spectrophotometric devices can be made either with a gas sample or with an in situ sensor which looks across the gas stack or through a short path length enclosed within a stack mounted probe. An example of an analyzing device using spectrographic techniques for the measurement or detection of various gaseous constituents is found in U.S. Pat. No. 3,805,196 which issued to Mr. J. D. Feichtner et al. on Apr. 16, 1974 and discloses the use of an acousto-optic tunable filter (AOTF). Depending on the geometry of the crystal from which the AOTF is manufactured and the RF signal that is used for modulation, the AOTF can be effective for detecting the absorption characteristics of specific gases through which an ultraviolet or infrared light beam is directed. Though this approach has proven very effective, it should be realized that to incorporate this approach into a workable arrangement requires additional detecting and support devices and can consequently become somewhat costly.
Still another approach to measuring SO.sub.2 can be found in U.S. Pat. No. 4,391,690 which issued to Mr. C. Y. Lin et al. on July 5, 1983. In this patent, a solid electrolyte electrochemical cell device specifically associated with SO.sub.2, provides an EMF signal based on the Nernst equation which indicates the SO.sub.2 content of the gas being monitored. With this device, an adjustment is necessary to eliminate the effect of oxygen on the EMF measurement signal. Although this approach is also effective, it would be advantageous if a device were available that could produce a measurement of SO.sub.2 in conjunction with some other function that such a device could perform.
An example of such a multipurpose measuring arrangement can be found in U.S. Pat. No. 4,394,240 issued to Mr. A. R. Pebler on July 19, 1983. Here, a combined sulfur oxide/oxygen measuring apparatus is provided wherein oxygen ion and oxy-anion conductive solid electrolyte electrochemical cells are combined to form a single gas measuring apparatus. An even more advantageous device than one which uses a combination of solid electrolyte cells would be one that could provide more than one reading using only one solid electrolyte cell.
Single purpose probes having a single solid electrolyte cell have been employed to measure oxygen in the gas stream of a combustible environment in such a manner that the oxygen analyzer is protected from damage that may occur from the contact of that analyzer with particulate matter such as fly ash, cinder, etc. An example of such a probe can be found in U.S. Pat. No. 3,928,161 which issued to Mr. W. H. McIntyre et al. on Dec. 23, 1975. This patent is assigned to the same assignee as the present invention and is hereby incorporated by reference to illustrate the use a solid electrolyte material as an oxygen analyzer, such material being, for instance, a composition of zirconia and oxides of calcium or related material which provides sufficient oxygen ion conduction.
An example of an industrial process where such an SO.sub.2 determining arrangement would be beneficial is in the area of a paper processing plant where sulfur burners are utilized, such sulfur burners which generate therefrom, high concentrations of SO.sub.2 as well as quantities of sulfuric acid which are used in the paper processing operation. Though the previous discussion has dealt with the measurement of SO.sub.2 from the standpoint of an emissions control operation, it should be realized that SO.sub.2 may be necessary for industrial operations such as chemical processing or paper mill operations for instance, and that accordingly, such measurement of SO.sub.2 is utilized as an operating guide or a closed loop control on a process control system where there are no emissions to air involved.